1. Field of the Invention
This invention relates to a process for producing a liquid polyalphaolefin homopolymer, e.g., 1-decene, or copolymer, e.g., one derived from 1-decene, employing hydrogen and a metallocene catalyst therefor, to the resulting polymer and to a lubricant composition in which the liquid polyalphaolefin functions as a viscosity modifier.
2. Description of the Prior Art
Catalytic oligomerization of olefins is a known technique for manufacturing hydrocarbon basestocks useful as lubricants. Efforts to improve upon the performance of natural mineral oil based lubricants by the synthesis of oligomeric hydrocarbon fluids have been the subject of important research and development in the petroleum industry for several decades, leading to recent commercial production of a number of superior poly(alphaolefin) synthetic lubricants (hereinafter referred to as “PAO”). These materials are primarily based on the oligomerization of alphaolefins such as C2-C20 olefins. Industrial research effort on synthetic lubricants has generally focused on fluids exhibiting useful viscosities over a wide range of temperature, i.e., improved viscosity index (VI), while also showing lubricity, thermal and oxidative stability and pour point equal to or better than mineral oil. These newer synthetic lubricants provide lower friction and hence increase mechanical efficiency across the full spectrum of mechanical loads and do so over a wider range of operating conditions than mineral oil lubricants.
Well known structural and physical property relationships for high polymers as contained in the various disciplines of polymer chemistry have pointed the way to alphaolefins as a fruitful field of investigation for the synthesis of oligomers with the structure thought to be needed to confer improved lubricant properties thereon. Due largely to studies on the polymerization of propene and vinyl monomers, the mechanism of the polymerization of alphaolefins and the effect of that mechanism on polymer structure is reasonably well understood, providing a strong resource for targeting on potentially useful oligomerization methods and oligomer structures. Building on that resource, oligomers of alphaolefins from 2 to 20 carbon atoms have been prepared with commercially useful synthetic lubricants from, e.g., 1-decene oligomerization, yielding a distinctly superior lubricant product via either cationic or Ziegler catalyzed polymerization.
A significant problem in the manufacture of synthetic lubricants is the production of lubricants in a preferred viscosity range in good yield without excessive catalyst deactivation. Frequently, it is difficult to directly produce lower viscosity range lubes without incurring lower yields due to the production of non-lubricant range materials. Methods to control molecular weight of lubricants in the oligomerization step are sought after in the art to overcome the problems in the manufacture of, particularly, lower viscosity lubricants.